The present invention relates to electro-optic imaging devices for display applications. More particularly, the present invention relates to a system and a method for varying the transmittance of light through a media on which images are displayed, based on non-conventional liquid crystal device (LCD) technology.
Head-up display systems (HUDs) and head-mounted displays (HMDs) are becoming increasingly widespread applications used for people under operational and time-dependent stresses and limitations. The displayed information is either of symbol graphics or alphanumerics for aeronautic pilot assistance during maneuver or combat tasks, or image displays for enhanced vision to assist aircraft landing or automotive drivers in adverse illumination or weather conditions. Among many other applications of such systems are included real-time physionometric data to be used by medical doctors and visual assembly instructions to be used by manufacturing workers.
The above-mentioned HUD and HMD systems utilize one or more combiner optical windows having see-through characteristics, which project display information superimposed on the background scene, thereby enabling the operator to perceive both the data and the background image without having to move his eyes and head back and forth between the display and the outside scene. Unfortunately, during outdoor applications, a background scene illuminated by sunlight is often of a very high brightness, in extreme cases approaching several thousand fL. Direct sunlight is even several orders of magnitude larger in brightness. If, for example, the displayed image has a brightness of 500 fL, the contrast of the displayed image against a background of 4000 fL will drop to about 11%. However, if the background brightness can be reduced to 800 fL, then the displayed image contrast will rise to about 40%.
There are two principal ways of reducing the background brightness: (a) utilization of a fixed, transparent shield of high optical density (as in sunglasses), and (b) using variable transmittance optics (VTO) with real-time adaptive transmittance. The introduction of a fixed shield according to method (a) is disadvantageous, because it will significantly reduce the visibility of the background scene under poor illumination. As a consequence, the VTO of method (b) is preferred, because it does not obstruct the view of the outside world in conditions of poor illumination.
Several technologies exist that could serve as VTOs, such as photochromic materials, electrochromic cells, and conventional LC technology. The photochromic materials suffer from some drawbacks, such as limited spectral and photochemical sensitivity, making most of these materials highly dependent on strong UV illumination which, with sunlight, prevails at noon time and is strongly reduced behind window shields. The electrochromic cells have very slow response times in the order of minutes, and hence they are not practical for fast-changing illumination conditions. Conventional LCD panels, sandwiched between polarizers and quarter-wave plates, have been proposed for welding filter VTO applications. Unfortunately, polarizers absorb an important part, over 50%, of the display illumination. Therefore, conventional LCD devices are impractical in HUD or HMD applications, because of the necessity to introduce polarizers which significantly reduce the brightness of the outside scenery and consequently are not acceptable to aeronautic or automotive users.
It is therefore a broad object of the present invention to overcome the disadvantages and limitations discussed above and to provide a system and a method for floating see-through image displays integrated with variable transmittance optical panels, such as windows or visors.
It is a primary object of the present invention to utilize suitable variable transmittance optical windows of non-conventional liquid crystal (LC) technology and materials, which can yield very high, maximal transmittance of light of over 50% and have low absorbance, reflectance or low scattering, which are of utmost importance in cases where the background scenery, as seen through the windows, is of poor illumination, so that the perceived background does not become one of poor visibility.
It is a further object of the present invention to utilize the same technology to adaptively reduce the transmittance of light through the VTO by a significant factor or contrast ratio of at least five, so as to provide sufficient image contrast of the display when it is viewed superimposed on a background object of very high brightness.
Another further specific objective of the present invention is to utilize suitable VTO technology, chosen from non-conventional, polarizer-free LC media including guest-host (GH) dichroic or pleochroic LCs and optimized GH polymer-stabilized cholesteric textures (GH-PSCT) or optimized GH surface-stabilized cholesteric textures (GH-SSCT), and/or combinations and derivatives of said non-conventional media. These media allow maximal light utilization because they minimize absorption losses from polarizers and do not exhibit scattering losses and thus allow large, maximal transmittance of light. Similarly, these media allow adaptive reduction in transmittance without negatively affecting the perceived background image, again because no additional light scattering is introduced which might reduce the background image contrast.
Yet another object of the present invention is to provide suitable contrast ratios so as to allow application of the proposed display devices in large ranges of background illumination conditions. This objective can be achieved by the configuration wherein two GH cells of mutually perpendicular alignment are stacked on top of each other. Such a configuration operates, as a matter of fact, as two orthogonal polarizers having a large range of variable transmittance of light, ranging from large extinction to maximal transmittance.
A still further objective of the present invention is to provide suitable light detector feedback electronic methods and systems for real-time VTO control of background brightness adaptation, so as to achieve constant information display contrast.
Another objective of the present invention is to provide methods for varying the transmitance of light over the VTO in a non-uniform manner, by applying mosaic structured cell assemblies having more than one electrical LC cell over the field of view (FOV) of the VTO, in conjunction with a multitude of light-sensor arrays and their corresponding electronic feedback. In such a mosaic or similar arrays, area-selective VTO action may be applied in order to selectively change the transmittance through the VTO in sub-areas of the FOV.
In accordance with the present invention, there is therefore provided a system for varying the transmittance of light through selected portions of a media on which images are displayed, said system comprising an image display panel; an image display source for projecting images on said media; LC-based, variable transmittance optical (VTO) media; a light sensor for measuring ambient light, and a signal generator connected to said VTO media for applying voltage to portions of said media at levels determined and controlled by said sensor, whereby the transmittance of light through said portions of said media is varied in accordance with the prevailing ambient light.
The invention further provides a method for varying the transmittance of light through selected portions of a media on which images are displayed, said method comprising providing an image display panel; providing a guest-host liquid crystal (GHLC) media, and applying voltages of selected levels to said portions of said GHLC media, depending on the prevailing ambient light, to effect changes in the transmittance of light through said portions of the media.